Which structure is specific to eukaryotic cells

ATP synthesis takes place on the inner membrane. Mitochondria have their own usually circular DNA chromosome that is stabilized by attachments to the inner membrane and carries genes similar to genes expressed by alpha-proteobacteria. Mitochondria also have special ribosomes and transfer RNAs that resemble these components in prokaryotes. These features all support the hypothesis that mitochondria were once free-living prokaryotes.

ATP represents the short-term stored energy of the cell. Cellular respiration is the process of making ATP using the chemical energy found in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product.

In fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a by-product. It is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP.

Your muscle cells need a lot of energy to keep your body moving. Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the production of lactic acid. In addition to the aerobic generation of ATP, mitochondria have several other metabolic functions. One of these functions is to generate clusters of iron and sulfur that are important cofactors of many enzymes.

Such functions are often associated with the reduced mitochondrion-derived organelles of anaerobic eukaryotes. There are two hypotheses about the origin of mitochondria: endosymbiotic and autogenous, but the most accredited theory at present is endosymbiosis.

The endosymbiotic hypothesis suggests mitochondria were originally prokaryotic cells, capable of implementing oxidative mechanisms. These prokaryotic cells may have been engulfed by a eukaryote and became endosymbionts living inside the eukaryote. Although they are both eukaryotic cells, there are unique structural differences between animal and plant cells.

Each eukaryotic cell has a plasma membrane, cytoplasm, a nucleus, ribosomes, mitochondria, peroxisomes, and in some, vacuoles; however, there are some striking differences between animal and plant cells. While both animal and plant cells have microtubule organizing centers MTOCs , animal cells also have centrioles associated with the MTOC: a complex called the centrosome. Animal cells each have a centrosome and lysosomes, whereas plant cells do not.

Plant cells have a cell wall, chloroplasts and other specialized plastids, and a large central vacuole, whereas animal cells do not. The centrosome is a microtubule-organizing center found near the nuclei of animal cells. It contains a pair of centrioles, two structures that lie perpendicular to each other. Each centriole is a cylinder of nine triplets of microtubules. The centrosome the organelle where all microtubules originate replicates itself before a cell divides, and the centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell.

The Centrosome Structure : The centrosome consists of two centrioles that lie at right angles to each other. Each centriole is a cylinder made up of nine triplets of microtubules. Nontubulin proteins indicated by the green lines hold the microtubule triplets together.

Animal cells have another set of organelles not found in plant cells: lysosomes. Enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. These enzymes are active at a much lower pH than that of the cytoplasm. Therefore, the pH within lysosomes is more acidic than the pH of the cytoplasm. Many reactions that take place in the cytoplasm could not occur at a low pH, so the advantage of compartmentalizing the eukaryotic cell into organelles is apparent.

The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell. Fungal and protistan cells also have cell walls. While the chief component of prokaryotic cell walls is peptidoglycan, the major organic molecule in the plant cell wall is cellulose, a polysaccharide comprised of glucose units.

When you bite into a raw vegetable, like celery, it crunches. The dashed lines at each end of the figure indicate a series of many more glucose units. The size of the page makes it impossible to portray an entire cellulose molecule. Like mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function.

Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen.

This is a major difference between plants and animals; plants autotrophs are able to make their own food, like sugars, while animals heterotrophs must ingest their food. The fluid enclosed by the inner membrane that surrounds the grana is called the stroma. The Chloroplast Structure : The chloroplast has an outer membrane, an inner membrane, and membrane structures called thylakoids that are stacked into grana.

The space inside the thylakoid membranes is called the thylakoid space. The light harvesting reactions take place in the thylakoid membranes, and the synthesis of sugar takes place in the fluid inside the inner membrane, which is called the stroma. They may also have other specialized functions depending on the cell type. Peroxisomes were first named for their ability to produce hydrogen peroxide, a highly reactive molecule that helps to break down molecules such as uric acid, amino acids, and fatty acids.

Peroxisomes also possess the enzyme catalase, which can degrade hydrogen peroxide. Along with the SER, peroxisomes also play a role in lipid biosynthesis. Like lysosomes, the compartmentalization of these degradative molecules within an organelle helps protect the cytoplasmic contents from unwanted damage.

The peroxisomes of certain organisms are specialized to meet their particular functional needs. For example, glyoxysomes are modified peroxisomes of yeasts and plant cells that perform several metabolic functions, including the production of sugar molecules. Similarly, glycosomes are modified peroxisomes made by certain trypanosomes, the pathogenic protozoans that cause Chagas disease and African sleeping sickness.

Eukaryotic cells have an internal cytoskeleton made of microfilaments , intermediate filaments , and microtubules. This matrix of fibers and tubes provides structural support as well as a network over which materials can be transported within the cell and on which organelles can be anchored Figure For example, the process of exocytosis involves the movement of a vesicle via the cytoskeletal network to the plasma membrane, where it can release its contents.

Figure The cytoskeleton is a network of microfilaments, intermediate filaments, and microtubules found throughout the cytoplasm of a eukaryotic cell.

In these fluorescently labeled animal cells, the microtubules are green, the actin microfilaments are red, the nucleus is blue, and keratin a type of intermediate filament is yellow. Microfilaments are composed of two intertwined strands of actin, each composed of actin monomers forming filamentous cables 6 nm in diameter [2] Figure The actin filaments work together with motor proteins, like myosin, to effect muscle contraction in animals or the amoeboid movement of some eukaryotic microbes.

In ameboid organisms, actin can be found in two forms: a stiffer, polymerized, gel form and a more fluid, unpolymerized soluble form. Actin in the gel form creates stability in the ectoplasm, the gel-like area of cytoplasm just inside the plasma membrane of ameboid protozoans. Once the cytoplasm extends outward, forming a pseudopodium, the remaining cytoplasm flows up to join the leading edge, thereby creating forward locomotion.

Beyond amoeboid movement, microfilaments are also involved in a variety of other processes in eukaryotic cells, including cytoplasmic streaming the movement or circulation of cytoplasm within the cell , cleavage furrow formation during cell division, and muscle movement in animals Figure These functions are the result of the dynamic nature of microfilaments, which can polymerize and depolymerize relatively easily in response to cellular signals, and their interactions with molecular motors in different types of eukaryotic cells.

Intermediate filaments Figure 12 are a diverse group of cytoskeletal filaments that act as cables within the cell.

Intermediate filaments tend to be more permanent in the cell and maintain the position of the nucleus. They also form the nuclear lamina lining or layer just inside the nuclear envelope. Additionally, intermediate filaments play a role in anchoring cells together in animal tissues. The intermediate filament protein desmin is found in desmosomes, the protein structures that join muscle cells together and help them resist external physical forces.

The intermediate filament protein keratin is a structural protein found in hair, skin, and nails. They are more permanent than other cytoskeletal structures and serve a variety of functions.

These form hollow tubes 23 nm in diameter that are used as girders within the cytoskeleton. Microtubules also work with motor proteins such as dynein and kinesin to move organelles and vesicles around within the cytoplasm. Additionally, microtubules are the main components of eukaryotic flagella and cilia, composing both the filament and the basal body components Figure Motor proteins carry organelles along microtubule tracks that crisscross the entire cell.

In addition, microtubules are involved in cell division, forming the mitotic spindle that serves to separate chromosomes during mitosis and meiosis. The mitotic spindle is produced by two centrosomes , which are essentially microtubule-organizing centers, at opposite ends of the cell. Each centrosome is composed of a pair of centrioles positioned at right angles to each other, and each centriole is an array of nine parallel microtubules arranged in triplets Figure Each centriole is composed of nine triplets of microtubules held together by accessory proteins.

The large, complex organelles in which aerobic cellular respiration occurs in eukaryotic cells are called mitochondria Figure Scientists during the s discovered that mitochondria have their own genome and 70S ribosomes.

The mitochondrial genome was found to be bacterial, when it was sequenced in These findings ultimately supported the endosymbiotic theory proposed by Lynn Margulis , which states that mitochondria originally arose through an endosymbiotic event in which a bacterium capable of aerobic cellular respiration was taken up by phagocytosis into a host cell and remained as a viable intracellular component.

Each mitochondrion has two lipid membranes. The inner membrane was derived from the bacterial plasma membrane. The electron transport chain for aerobic respiration uses integral proteins embedded in the inner membrane. It also contains mitochondrial DNA and 70S ribosomes. Invaginations of the inner membrane, called cristae, evolved to increase surface area for the location of biochemical reactions.

The folding patterns of the cristae differ among various types of eukaryotic cells and are used to distinguish different eukaryotic organisms from each other. Each mitochondrion is surrounded by two membranes, the inner of which is extensively folded into cristae and is the site of the intermembrane space. The mitochondrial matrix contains the mitochondrial DNA, ribosomes, and metabolic enzymes. The transmission electron micrograph of a mitochondrion, on the right, shows both membranes, including cristae and the mitochondrial matrix.

Photosynthesis takes place in chloroplasts, which have an outer membrane and an inner membrane. Stacks of thylakoids called grana form a third membrane layer. Plant cells and algal cells contain chloroplasts , the organelles in which photosynthesis occurs Figure All chloroplasts have at least three membrane systems: the outer membrane, the inner membrane, and the thylakoid membrane system. The thylakoid system is a highly dynamic collection of folded membrane sacs. It is where the green photosynthetic pigment chlorophyll is found and the light reactions of photosynthesis occur.

In most plant chloroplasts, the thylakoids are arranged in stacks called grana singular: granum , whereas in some algal chloroplasts , the thylakoids are free floating. Other organelles similar to mitochondria have arisen in other types of eukaryotes, but their roles differ.

Hydrogenosomes are found in some anaerobic eukaryotes and serve as the location of anaerobic hydrogen production. Hydrogenosomes typically lack their own DNA and ribosomes.

Kinetoplasts are a variation of the mitochondria found in some eukaryotic pathogens. In these organisms, each cell has a single, long, branched mitochondrion in which kinetoplast DNA, organized as multiple circular pieces of DNA, is found concentrated at one pole of the cell. Many protozoans, including several protozoan parasites that cause infections in humans, can be identified by their unusual appearance.

Distinguishing features may include complex cell morphologies, the presence of unique organelles, or the absence of common organelles. The protozoan parasites Giardia lamblia and Trichomonas vaginalis are two examples. Its Golgi apparatus and endoplasmic reticulum are greatly reduced, and it lacks mitochondria completely. However, it does have organelles known as mitosomes , double-membrane-bound organelles that appear to be severely reduced mitochondria. What are eukaryotic cells?

Why do prokaryotic cells have no nucleus? How does dna differ in eukaryotic and prokaryotic cells? How do prokaryotic and eukaryotic cells differ? Eukaryotic Cells. Figure 1: A mitochondrion. Figure 2: A chloroplast. What Defines an Organelle? Why Is the Nucleus So Important? Why Are Mitochondria and Chloroplasts Special? Figure 4: The origin of mitochondria and chloroplasts.

Mitochondria and chloroplasts likely evolved from engulfed bacteria that once lived as independent organisms. Figure 5: Typical prokaryotic left and eukaryotic right cells.

In prokaryotes, the DNA chromosome is in contact with the cellular cytoplasm and is not in a housed membrane-bound nucleus. Figure 6: The relationship between the radius, surface area, and volume of a cell. Note that as the radius of a cell increases from 1x to 3x left , the surface area increases from 1x to 9x, and the volume increases from 1x to 27x. Organelles serve specific functions within eukaryotes, such as energy production, photosynthesis, and membrane construction.

Most are membrane-bound structures that are the sites of specific types of biochemical reactions. The nucleus is particularly important among eukaryotic organelles because it is the location of a cell's DNA.

Two other critical organelles are mitochondria and chloroplasts, which play important roles in energy conversion and are thought to have their evolutionary origins as simple single-celled organisms. Cell Biology for Seminars, Unit 1. Topic rooms within Cell Biology Close. No topic rooms are there.

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